From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge-Transfer Phase Transition

We study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb_0.94Mn_0.94Co_0.06[Fe(CN)₆]_0.98 ⋅ 0.2H₂O material, induced at room temperature by a single laser shot. This system exhibits a charge-transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn³⁺-N-C-Fe²⁺ tetragonal phase to the high temperature Mn²⁺-N-C-Fe³⁺ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out-of-equilibrium dynamics leading to this phase is multi-scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N-C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn³⁺-N-C-Fe²⁺ phase creates small charge-transfer polarons [Mn²⁺-N-C-Fe³⁺]* within ≃250 fs. The local trapping of photoinduced intermetallic charge-transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn²⁺-N-C-Fe²⁺ species. Above a threshold fluence, when a critical fraction of small CT-polarons is reached, the macroscopic phase transition to the persistent Mn²⁺-N-C-Fe³⁺ cubic phase occurs within ≃ 100 ps. This non-linear photo-response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.